1. Field of the Invention
This invention relates to a thermal head for use in a heat sensitive type printer or heat transfer type printer, as well as to a method of manufacturing the same. More specifically, this invention concerns an improvement in the undercoat layer
2. Description of the Prior Art
The conventional thermal head has a cross sectional structure, for example, as shown in FIG. 2 prepared by using a partially glazed substrate in which a glazed layer 2 is partially formed on the surface of an insulating substrate 1 such as made of alumina and by laminating thereover an undercoat layer 3' such as made of a Ta.sub.2 O.sub.5 film, a heat generating resistor layer 4 such as made of a Ta.sub.2 N or Ta-W-N film, an electric power supply conductor layer 5 such as made of an Al film and a protection layer 6 such as made of a dual layer film composed of a lower SiO.sub.2 layer film and an upper Ta.sub.2 O.sub.5 layer film successively. A part of the heat generating portion 7 generates heat to provide heat energy for developing color on heat sensitive paper 8 or the like in contact therewith.
In this case, the undercoat layer 3' is inserted with an aim of preventing the glazed layer 2 from being etched simultaneously upon etching the heat generating resistor layer 4 formed just above. That is, it is necessary to use a mixture of fluoric acid and nitric acid as a etchant for etching the heat generating resistor layer 4 composed, for example, of a Ta.sub.2 N or Ta-W-N film to form a pattern. Since the etchant rapidly erodes the glazes layer 2 made of glass, peeling is resulted to the heat generating resistor film at the edged portion of the heat generating resistor pattern to cause the abnormality in the pattern configuration. A thermal head having the heat generating resistor pattern of such an abnormal shape involves a problem such as increase in the resistance value and shortening in the heat generating life. Then, for overcoming the problem, a film less sensitive to the etchant is formed as the undercoat layer 3' over the glazed layer 2 thereby protecting the layer 2. As the material for the undercoat layer 3', a Ta.sub.2 O.sub.5 film with an extremely low etching tendency to the fluoro-nitric acid has usually been employed.
However, the conventional thermal head comprising the undercoat layer 3' composed of the Ta.sub.2 O.sub.5 has various problems as described below.
FIG. 3(c) shows the result of the step-stress test (SST) for the conventional thermal head as described above. In this case, SST is a sort of acceleration tests for evaluating the heat-resistant stability of the thermal head, in which an appropriate pulse voltage is applied for a predetermined of time to the heat generating resistor body to measure the variation relative to the initial resistance value and the variation coefficient for the resistance value in each of the steps is plotted while gradually increasing the application voltage till the heat generating resistor body is burnt to be disconnected. If the variation coefficient for the resistance value increases in excess of 20%, the heat generating resistor body is judged to be destructed.
Now, FIG. 3(b) shows the result of the SST for the conventional thermal head having a plurality of heat generating portion (dots). As can be seen from the figure, destruction already appears at a relatively low electric power applied and there is a drawback that the destruction voltage varies depending on the dots. Generation of dots destructed at such a low electric power applied significantly degrades the reliability of the thermal head to result in a significant problem.